Method for removing mercaptan sulphur from petroleum distillate



Patented Dec. 5, 1944 METHOD FOR REMOVING MERCAPTAN SUL- PHUB FROM PETROLEUM DISTILLATE Jan G. Schaafsma, Pasadena, Calif., assignor to.

Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York No Drawing. Application my 25, 1942. Serial m. 4,494

scam.

The object or the invention is to provide a method for removing a portion of the sulphur from sulphur-containing petroleum distillates by treatment in the vapor phase with solid alkaliearth metal hydroxides.

It is known that the mercaptans (alkyl hydrosulphids) which occur in the lighter disthiates from many sulphur-containing petroleums are reactive with the alkali-earth metal hydroxides to form the sulphid of the metal and the hydrocarbon corresponding to-the alkyl radical of the mercaptan.

The attempts heretofore made to utilize this reaction for the commercial removal of mercaptans from light petroleum distillates (the opera-' tion known as sweetening) have taken the form of adding the oxide or the hydroxide to the liquid undergoing distillation in batch or tube stills. This addition, while useful in combating some types of corrosion of equipment, has proven wholly ineffective as a means for reducing the sulphur content of the distillates produced, or of obtaining a sweet distillate. Even at temperatures from 750 to '900' Fahr. there is substantially no conversion of mercaptan sulphur to metallic sulphid and, consequently, no appreciable reduction in the over-all sulphur content of the distilled product.

I have discovered that the above named reaction, which fails in the liquid phase and in the presence of a large excess of liquid hydrocarbon, may be produced with ease and at high velocity in the vapor phase. I have further discovered that by applying the alkali-earth metal hydroxide to the mercaptan-containing vapors, with the observance of certain limiting conditions which will be described, the mercaptan sulphur content of petroleum distillates of the gasoline type may be almost or completely removed at a very moderate cost for materials, manipulation, and equipment.

The method has the advantage over the conventional oxidizing methods of sweetening that the sulphur contained in the mercaptans is actually removed from the distillate instead of being retained in the form of organic sulphids.

In carrying out the process of the invention I pass the mercaptan-containing vapors through a bed of pulverulent or granulated and at least partially slakcd lime at (preferably) superatmospheric pressure and an elevated temperature. Lime is hereafter referred to in this description as a type of the alkali-earth metal oxides or hydroxides, as while all of the latter are functional in this process, only the calcium compounds are at present commercially available from the cost standpoint.

The lime must be at least partially hydrated. The effectiveness of the lime for this purpose apof hydration. As parallel experiments using comminuted or granular quicklime (calcium oxide) failed to produce any appreciable result, it may be assumed that only the calcium hydroxide content of a partially slaked lime is utilized. It is therefore most economical on the score of lime consumption to operate with fully hydrated lime (24.3% water) though for facility in preparing the .lime bed (an appreciable factor in the total cost of treatment), it may be expedient to use a lime only partially slaked, with some loss in efiiciency.

The physical condition of the lime must be such as to permit the vapor stream to pass through the bed with a reasonable pressure drop and without anymaterial carry-over of solids. As complete slaking reduces quicklime to an impalpable powder (at least in part), I have recourse to three alternative methods for forming the material of which the bed is to be composed. The first is to slightly dampen a completely slaked lime and then pelletize it under pressure. The second is to completely slake the lime with enough water to form a paste, dry at say 220 Fahr., crush to the desired size and screen out the fines. Both of these methods yield a completely hydrated product having the maximum efiiciency. The third method is to start with the quicklime in granular form, air-slake to a point just short of material break-down of the granules, and separate any dust thus formed. Granular quicklime may be slaked in moist air, to about 16% or 17% water without much deprmiation of the granular form.

If the hydroxide is to be aformed into vaporpermeable beds, it is desirable to use the smallest particle size which is permissible in view of the dimensions of the bed and the predetermined velocity of vapor through it. I have found a suitable size range for average use to be such as will pass through a l0-mesh screen and be retained on a screen having 30 meshes to the linear inch. It will be understood that this is a physical limitation only and that the sole requirement for functionality is contact between the lime particles and the vapor. however it may be produced. I therefore contemplate as functional, though perhaps not commercially practical, methods in which finely comminuted lime is passed over a succession of trays or bafiles in counterflow to the vapor or even methods in which the lime powder is dusted down through a slowly ascending vapor column.

In the use of granular lime it is fully equivalent to make use of stationary lime beds which are renewed as they become fouled and inert or to feed fresh lime onto the top of a gradually subsiding bed with continuous or periodical removal of spent lime from the lower end.

pears to be a straight line function of the deg ee The temperature of the lime bed should be maintained within the approximate limits 800 to 000' Fahr. to secure the best results. The removal of mercaptans is accelerated by elevation of temperature and below about 800' Fahr. is likely to be incomplete while above about 000 Fahr. the rate of fouling due to side reactions exceeds the rate of sweetenins. The optimum temperature in any specific instance can be determined only by fiuctuatlns the-temperature and oboervins the effects thus produced. It will be found that the process is quite sensitive to temperature control and when the optimum temperature' has been found, it should be maintained within a relatively narrow range, not exceeding 25 Fahr.

The pressure carried is not critical but experiments indicate that as the contacting pressure diminisheatbesiseoftheapparatusandthe quantity of lime required to produce satisfactory sweetenina increase out of proportion with theincreaseinvaporvolume. Ontheother hand, as pressures increase the strength and the first cost of the apparatus are increased. Thus the optimum operating pressure in any siven instance will be determined by oflsetting apparatus costs and capacities rather than by any functional limitations. The method may be operated successfully at pressures ransing from atmospheric to at least 100 pounds gauze and probably even hisher.

Inmyobservationthe foulingofthelimebed is dependent to a far greater extent on the time during which the lime is contacted with vapor than on the quantity of sulphur which it has absorbed. For this reason the mom emcient utilization of the lime is promoted by raising the vapor velocity through the bed to the maximum at which a sweet product is realized. In the best practice, however, only a fraction of the totalquantityoflimeinthebedisactually utilized. As the spent lime contains a considerable proportion oi calcium sulphids, it cannot be regenerated by burning and may usually be discarded.

After a given lime bed has become fouled to the point at which it will no longer yield a sweet distillate at the desired vapor velocity, it still retains the ability to react with considerably more mercaptan sulphur. The maximum economy in lime consumption is therefore attained only in theuse ofalimebedmoving incounterfiow to the vapor and of such lensth that completely spent lime only need be removed from the end at which the vapor enters. An approach to the same eiiectiveness may attained in the use of a series of two or more stationary beds, the vapor beins passed first throush a partially spent bed and then throuah a bed of relatively fresh lime. In such series the eifectiveness increass, at a diminishing rate, as the number of contact beds in series increases. I have obtained sood results, to wit a reduction in reagent consumption to below that obtained in a sinsle chamber, by the use of two chambers in series for the reaction and a third being discharged and recharsed.

The treatment of petroleum vapors in the stated temperature range is accompanied by a small amount of crackina, the liquid recovery bolus of the order of 98% to 9 of the volume charred. The separation of the gas thus produced must be cared for in the condensins arrangements. The polymerization of unsatua,se4,sao

tactins of petroleum vapors with aluminum ox- I ides and silicates has not been observed when calcium hydroxide is used as the contact solid.

A method of slaking the lime which avoids any operation extraneous to the contactor is to feed the quicklime, in sranular form, onto the top of the treating bed and to slake it in situ by means of superheated steam diluted with an inert gas or, better, with sweetened sasoline,

vapors. In this operation it is necessary to have the lime bed extend above the level of the vapor outlet and to introduce the diluted steam through a distributor which will spread it over the upper surface of the bed and cause it to contact the unslaked lime as completely and evenly as possitale. The temperature of the diluted steam should be substantially that of the outfiowing sweetened vapor and the dilution of the steam must be so controlled as to produce substantial hydration of the lime without causing it to cake.

Thetable followinssetsforththeresults otthe above described operation when applied to a viscosity-breaker pressure distillate under optimum conditions as to temperature and pressure. The fisures in column 1 show the characteristics of the stock charged. Column 2 shows the operating conditions in and the results of a treatment in which a substantially completely slated limewasusedinasingleehamber. column; lives similar figures for a treatment using partially slated lime in a sinsle chamber. Column 4 gives similar figures for a treatment in which partiallyslakedlimewasusedinaseriesottwo chambers.

1 Column No Watarin limsaseharged. -.-percant its 16.0 it. Mesh of lime as charged. 10-3 10-1 ill-I Number oi chambers in arias 1 l 1 Operating tempcatur Iain M an ID Operating preastm.....poundsgauga I) so I Space velocity 1 8. 02 8. 0i 1 0.10 Length of run h urs 1! 18 0.78

mum neat MIG Bar-rails raw stock per ton lime as til m as PoundsCaOperbarnirawstot 1.0 as 3.7

Yields band as m does I Volume liquid product "plane I. 6' I. 0 l 0 Gas, Hi8 iroe..-.oubioiastperbanal ll.l 11.0 11.!

Properties steel sad prod Reed vapor pre-ure "pounds" 0. 7 l0l 0 l0. 0 10. d A. P. I. gravity degrees.. 02.7 02.! 010 Initial boiling point .do.... 0i 0! 01 07 End point .do.... 867 I78 870 I70 Gums, cop dish 23 I4 I03 ll Gums, A. T. M-- 7 6 2 I Total sulphur content.. percent... 0. 0d 0. 46 0. l0 0. It Mercaptsn sulphur content -.de 0. 1d a one 0. ill! 0. Octane number, motor method 70. 5 70. 7 71. 0 70. 7 Octane number, M. M.+l oe. TEL... 71. 6 74. ii 76. 1 1* I Octane number, M. M.+8 cc. TEL... 7d. 1 77. 7 78. 2 7&0 Octanenumbmlitireaaarobmath 78.! 70.4 710 70.8

Nd recalls Relation mereaptaa sulphur to totgl 1 a i or pare-l Reduction in total sulfur. do I I a: OctanenumberinmlLM. clear 0.! 0.6 0.3 Octane number 11. I. I so.

- 1.0 I. l L.

l Volume vapor pa bowls-volume lime. I lured on line in see-d (pd) dambc rates which occurs in the high temperature con- "ll drated calcium oxide in granular form. said bed being maintained at a temperature between 800 and 900 Fahr.; withdrawing spent granules from the side or said bed at which said vapor enters; introducing substantially anhydrous calcium oxide in lump form to the side of said bed at which said vapor emerges, and reducing said lumps to the form of partially hydrated granules by the controlled admission of steam at the side at 10 whichsaid lumps are introduced.

JAN GQ SCHAAFSMA. 

